In the weeks since a Florida high school shooting, national retailers have moved to voluntarily stop selling firearms to those under the age of 21. But those store policies might violate state laws against age discrimination, according to a firearms industry trade group.

Walmart Inc., Dick’s Sporting Goods Inc., Kroger Co. and L.L. Bean Inc. all announced they would stop selling firearms to 18- to 20-year-olds in the wake of the Parkland, Florida, high school shooting, in which a 19-year-old gunman was charged with killing 17 people. Orvis Co., an outdoor goods retailer, said it would sell firearms only to those under the age of 21 if they complete a education and safety course.

The National Shooting Sports Foundation warned on its website Thursday that licensed gun dealers who refuse to sell to customers under 21 “may violate state or local laws barring age discrimination” and could face the prospect of “civil lawsuits or civil enforcement actions.” The message on the trade group’s website was addressed to federally licensed firearms retailers, also called FFLs. That category includes the nation’s gun dealers, ranging from mom and pop shops to such nationwide retailers as Walmart.

At least nine states cited by NSSF, including Delaware, Louisiana, Michigan and Oregon, have laws on the books related to age discrimination. Washington D.C. and New York City have comparable laws.

There have already been attempts to challenge the retailers’ new age-related policies with litigation. A 20-year-old Oregon man, Tyler Watson, filed an age-discrimination lawsuit against Dick’s Sporting Goods and Walmart, according to a local news report. He attempted to purchase rifles from Walmart and a Field & Stream store, a chain owned by Dick’s. Both stores declined to sell him the rifles, citing the recent change in policy. A teenager in Michigan filed a similar suit against Dick’s.

A spokesperson for Walmart said, “We stand behind our decision and plan to defend it.” Dick’s Sporting Goods did not reply to immediate request for comment on the matter.